The various architectures of radio receivers find their origins in exceptional is innovations of the early part of the 20th century; some indeed can be traced to the latter years of the 19th century. Radio communications are based upon the observation that electro-magnetic energy is capable of radiating into empty space as a constant exchange of magnetic and electrical energy. An oscillating electrical signal in a conducting material creates such radiation, to a greater or lesser degree dependent upon the so called “matching” of the conducting material to the frequency of oscillation, and when such radiation impinges upon another suitable material it induces current or voltage oscillations in that material, again to a greater or lesser extent depending on the matching. This induced signal is very small, as perhaps can be appreciated by the fact that the solid angle subtended by the receiving material relative to the transmitting material can be very small indeed, and so the percentage of energy collectable in even an ideal system is tiny.
The physicist is inclined to consider the operation of a radio receiver as a thermodynamic device, inquiring into the total energy representing the signal at the receiver, and asking to what degree this signal exceeds the thermal noise present due to the finite temperature of the environment. All energy storage means are coupled to the ambient temperature of the environment as is well understood from the 19th century study of thermodynamics. Hence they all acquire a certain amount of energy within a given bandwidth and the wanted signal is additive to this thermal noise. The extent to which the signal exceeds the noise is the signal-to-noise ratio.
Thermal noise exists in equal amounts across all possible frequencies of the oscillation of voltage or current. Indeed, one of the outstanding conclusions of thermodynamics is the ‘equipartition of energy’ theorem which states that there will be an equal quantity of energy in each possible bandwidth of radio receiver. Hence the total magnitude of thermal noise is assessed by summing over all frequencies of signal that are present at the receiver. This last observation of the physicist is key to the engineer's design of a radio: the signal can be distinguished from the noise to a much higher degree if the bandwidth of the received signal is restricted. That is to say, if the engineer is able to make the radio receiver neglect all those frequencies that are not part of the desired signal, then the total noise will be less, since the noise must be added for every frequency present at the receiver output. This aspect of the radio is called its “selectivity” and is critical in the separation of the signal from the noise. Higher selectivity means higher performance, lower noise, and better fidelity. We can view the progress of radio design from the late 19th century to early 21st century as a continual improvement of selectivity in particular, and a deepening understanding of noise in a communication channel in general.
How then is selectivity achieved in a radio receiver? The answer is via use of electronic amplification of certain narrow bandwidths of interest and suppression of out-of-band frequencies. The optimum elemental device to achieve this is the tuned circuit: a combination of inductance and capacitance that exhibits simple harmonic motion as energy is exchanged from the electric field of the capacitor to the magnetic field of the inductor and vice-versa. Consequently, the conceptually simplest viable radio receiver is of the so-called “tuned RF” type that arranges for a high Q circuit to amplify the desired signal frequency as much as possible and rapidly roll off its response to signals of any other frequency. However, the tuned RF receiver suffers from a practical difficulty: in order to change the received frequency this tuned circuit must be changed, involving somehow adjusting the value of the inductor and/or capacitor, a task that is not impossible, but is inconvenient and somewhat costly. It would be far more convenient and less costly if the selectivity were always provided at a fixed frequency; a seemingly to impossible task since the received frequency is required to change as different radio stations are selected.
This conundrum is solved by the heterodyne principle invented in the latter years of the 19th century, whereby the desired power of the received signal is shifted in frequency to a constant intermediate frequency and selectivity is achieved by circuit elements working at this fixed intermediate frequency. Specifically, the received signal at the desired frequency of reception is caused to interact with a signal generated locally in the receiver in such a way as to produce a third frequency, the intermediate frequency. In fact, the heterodyne principle necessarily generates two output frequencies: one at ‘frec+flo’ and one at ‘frec−flo’ where ‘free is the received frequency and to’ is the local oscillator frequency. The “interaction” of local oscillator and received signal is ideally multiplication and the electronic element that achieves this multiplication is called a “mixer”.
It is further arranged that the locally generated signal, the local oscillator, is adjustable in frequency of oscillation and so is able to transform any of a range of received frequencies to the same intermediate frequency. Selectivity is provided at the fixed intermediate frequency by components that need not change; the choice of frequency to receive is set by the adjustable local oscillator. This heterodyne principle, or more precisely a variant of it that selects only one of the two heterodyne frequencies (typically the difference frequency ‘frec−flo’), the so called “super-heterodyne” receiver, has come to dominate: it is ubiquitous in all applications of radio communication.
But, this domination of the super-heterodyne radio comes at a cost in a common class of radio applications: there are situations where two distinctly different received frequencies generate the same intermediate frequency. That is, there are circumstances where the super-heterodyne radio cannot resolve a single received frequency and so is susceptible to unwanted interference from a second radio transmitter if it happens to be at that second frequency. This inseparable second frequency is commonly called the “image frequency” and the super-heterodyne must be augmented by additional circuitry to discriminate between the image and the real received frequency.
This augmentation turns out to be far from trivial and its solution had to await the is invention of the “Single Sideband Modulator” by Hartley in 1925 (see U.S. Pat. No. 1,666,206) which, although directed to a somewhat different task, nevertheless showed the principle by which the image may be removed after the mixing process. The Hartley image rejection receiver operates by recognizing that the phase of the image and the real frequencies differ, and, by use of a suitable phase shifting network, illustrated in FIG. 1, a vector-sum cancellation may be achieved that removes the image. Consequently, if that “suitable phase shifting network” can be made we have removed the remaining obstacle to the universal use of the super-heterodyne receiver.
As the reader may have guessed, the “suitable phase shifting network” is itself not perfect: it is probably impossible to make a fixed phase shift over more than a narrow range of frequencies and the Hartley image rejection method is limited by the difficulty of making a wide frequency range phase shifting circuit.
In 1956 Donald Weaver described what has become known as the Weaver architecture for image rejection in his paper entitled “A third method of generation and detection of single-sideband signals”, D. K. Weaver—Proc. IRE, 1956. The first method implied in Weaver's title is the filter used prior to Hartley; the second method is Hartley's method. Illustrated in FIG. 2, Weaver's method relies upon two sets of mixers; each set is called a quadrature modulator because it multiplies the signal by a local oscillator that has not one but two outputs. The two outputs are in quadrature to each other—they are 90 degrees apart in phase. Through a series of trigonometric identities Weaver shows that a frequency shift as required in the heterodyne receiver can be accomplished with the additional feature that the image frequency is removed. Clearly set out in his paper, the effect of two sets of quadrature mixers in sequence are shown in mathematical detail and the result is inescapable: assuming the mixers are indeed in quadrature and that the intermediate signals are filtered to remove the upper of the mixed frequencies then no image can be present in the output. Unlike the Hartley approach, the Weaver circuit is implementable without is approximation, and it operates over unlimited bandwidth since no phase shift network is needed.
Weaver's architecture has been successful. Modern digital radios (cell phones, TV receivers, etc.) use his architecture and achieve image rejection to an adequate degree. The Weaver architecture, limited only by the quadrature nature of the signals, can reject an image by about 50 db (about 1 part in 300). Various digital enhancements can, with advanced signal processing in the digital domain, improve image rejection to perhaps 60 db (1 part in 1000).
However, a reading of Weaver's paper will show that his architecture requires that the signal flowing from the first set of mixers to the second must be filtered: the sum frequency must be removed. This signal flow is in two parts, one signal flows from one of the first mixers to one of the second mixers, and a second signal flows between the remaining two mixers. It is within this dual signal path that the required filters must be placed to block the sum frequency. Recall that the image rejection of the architecture relies upon the quadrature nature of the signals—they must be 90 degrees apart—to a high degree. Even as little as a one degree phase difference will limit the ultimate image rejection to less than 60 db. It is quickly realized therefore that the two filters that are required to be placed in the two signal paths must match such that the phase difference between them is substantially less than one degree to retain the best image rejection. This is difficult: the two filters will limit the image rejection unless they match to a very high degree. A means of overcoming this limitation is the subject of the present disclosure.
If an analog circuit could create a matched pair of filters, then the relative expense and power consumed by the only other known solution to improve matching, namely conversion into the digital signal processing domain, could be avoided.
This disclosure first teaches how to make a Weaver architecture radio where no filter is required between the first and second set of mixers. Hence no phase error is incurred and image rejection is substantially higher than can be achieved by any analog implementation of the conventional Weaver architecture.
In addition to the absence of filters, this disclosure documents a second innovation: the use of a time-division multiplexed second set of mixers, again aimed at the improvement of image rejection.
It is clear that matching of the second set of mixers as well as the filter matching will affect image rejection—the second set of mixers may not be precisely in quadrature to each other. This is why many modern radio receivers place ADC's (analog to digital converters) after the first mixers and accomplish the second mixing in the digital domain. Once in the digital domain algorithms exist to digitally correct any phase difference in the two paths. In a digital implementation it is possible to time-division multiplex since the state of the digital filter can be restored upon switching the multiplexer. In Weaver's architecture implemented in the analog domain it is not possible to time-division multiplex the second set of mixers due to the need for filtering because time division multiplexing destroys the action of the filter. The filter sees a high speed signal as it must handle the two signals in sequence. Therefore, the signal seen by the filter is not the actual first mixer output—it is a chopped version of it. This cannot be filtered without maintaining separate state-variables for each part of the time division multiplexer. However, when the first innovation is employed no filter is necessary—and in this case time-division multiplexing in the analog domain can be used. This enables the design of a single circuit representing both of the second mixers, wherein no phase error is introduced and image rejection is enhanced.
The objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows. Note that not all of the objects or advantages implied by this disclosure are necessarily met by all embodiments of the invention described below or by the invention defined in each of the claims.